1. Field of the Invention
This invention relates to a semiconductor ceramic composition of a barium titanate system and, more particularly, to a semiconductor ceramic composition with a positive resistance temperature characteristic, which has a low specific resistance, a high breakdown voltage and an excellent inrush-stability.
2. Description of the Prior Art
It is well known that semiconductor ceramics of a barium titanate system, comprising barium titanate doped with a small amount of one or more semiconductor-forming elements selected form the group consisting of oxides of Nb, Bi, Sb, W, Th and rare earth elements such as Y, La and Ce, have a positive temperature coefficient of resistance and possess an abrupt positive change of resistance at a temperature over a curie point. In practical applications, however, the semiconductor ceramics with a positive resistance-temperature characteristic are required to have other physical and electrical properties such as, for example, a desired curie point, low specific resistance at ordinary temperature, a large temperature coefficient of resistance at a temperature over the curie point, a high breakdown voltage, and good inrush-switching stability. For this reason, various modifications of semiconductor ceramics have been proposed in various patents, for example, U.S. Pat. Nos. 3,975,307 and 4,096,098 to improve the above properties. For example, it has been proposed to replace a part of the barium in barium titanate with a lead to shift the curie temperature from the normal curie point of 120.degree. C. to a higher temperature. Also, it has been proposed to replace a part of the barium with Sr or replace a part of the titanium with Zr or Sn to shift the curie point to a lower temperature.
On the other hand, to improve the resistance change rate at a temperature over the curie point it has been proposed to incorporate Mn in an amount of 0.03 to 0.15 mole % into the composition. Also, It has been proposed to decrease the specific resistance at ordinary temperature by the addition of SiO.sub.2 in an amount of 0.5 to 5 mol %.
Such modified semiconductor ceramics can be utilized in positive temperature coefficient(PTC) thermistors for use as thermostat heating elements, current control elements, thermal control elements and the like.
However, such conventional semiconductor ceramic compositions provide a narrow application flexibility so that semiconductor ceramics for one purpose cannot be used for other purposes. Also, the conventional semiconductor ceramics have a serious drawback awaiting a solution. For example, the lead-modified barium titanate semiconductor ceramics have been utilized in current control elements in starting circuits for motors, but these current control elements have a tendency to break stratiformly. In the starting circuits, a large inrush current flows for a relatively long time. This problem can be solved by replacing a part of the barium in BaTiO.sub.3 with lead and calcium and incorporating silica and manganese into the basic composition. Such semiconductor ceramics cannot be used for various purposes, but only to current control elements for use in the starting circuits for motors.
It has been reported that barium titanate semiconductor ceramics modified with calcium or with calcium and strontium, and containing Mn and SiO.sub.2 possess low specific resistance, less than 10 ohm-cm, at ordinary temperature and have a dielectric breakdown voltage of 48 V/mm at the maximum. However, this breakdown voltage is too low to put the semiconductor ceramics into practical use, and they are poor with respect to the inrush-switching stability when there is applied an AC inrush voltage.
Also, semiconductor ceramics of a barium titanate have been proposed in which a part of the barium in BaTiO.sub.3 is replaced with lead and strontium, and Mn and SiO.sub.2 are incorporated therein. Such semiconductor ceramics exhibit excellent breakdown voltage characteristics and high-temperature load characteristic, but it is impossible to obtain an excellent inrush-switching stability.
U.S. Pat. No. 4,014,822 discloses semiconductor ceramic composition consisting essentially of 83.95 to 98.65 mol % of barium titanate, 0.13 to 0.45 mol % of manganese oxide, 0.20 to 15 mol % of silicon dioxide, and 0.23 to 1.65 mol % of one or more oxides of rare earth elements, bismuth and antimony, and the content of said one or more oxides being 0.10 to 1.20 mol % greater than the content of manganese oxide. Also, it discloses replacement of a part of the barium in BaTiO.sub.3 with one or more oxides selected from the group of strontium oxide, calcium oxide and lead oxide. Such semiconductor ceramics have high positive resistance temperature characteristic, small dependency of resistance on voltage, and excellent high-voltage resistant life characteristic. However, they have a disadvantage that an increase in the breakdown voltage and inrush-switching stability increases a specific resistance at ordinary temperature considerably.
It has now been found that the joint addition of Pb, Sr and Ca to the barium titanate semiconductor ceramics composition enables to produce semiconductor ceramics which can be used for various purposes such as, for example, as positive temperature coefficient thermistors for use in motor-starting circuit, degausing circuits, thermostat heating devices and the like.